Having the same problem man... Was living in an apartment complex and had no option but to keep the car outside. It got rained on and the sprinkler system in the complex always hit the one side of the car in the morning, leaving the sun to dry off the side (and leaving spots all up and down the side and on the roof). Washing it just doesn't do the trick, and sometimes seems to make it worse!

I can vouch for the white vinegar and claybar methods. both worked and I have even done some steel wool on really tough spots but only on windows. I told a colleague about the steel wool and he used it on paint... dope...

Steel wool definitely works as long as the spots haven't been on there a long time. Depending on how bad the spots are, it's possible that nothing short of having the windows professionally polished will take them off. Vinegar and sprays only take off mild spots at best.